1. Field of the Invention
The present invention relates to a recording apparatus for printing various types of data, and more particularly, to an ink-jet recording apparatus.
2. Description of the Related Art
So-called "ink-jet recording method" is known, in which droplets of liquid ink are ejected onto a recording medium, thereby forming dots thereon.
The ink-jet recording method accomplishes data recording, making less noise than other recording methods. Furthermore, the method does not involve such processes as development and fixing. Because of these advantages, the ink-jet recording method has received a great deal of attraction as technique of recording data on plain paper.
Various ink-jet recording methods have been devised and disclosed. Among them is so-called "slit jet recording method," in which a number of recording electrodes juxtaposed along an ink-ejecting slit are used to control the ejection of ink droplets. The slit jet recording method is regarded as desirable, since it achieves high-speed recording and scarcely cause ink clogging.
An electrostatic acceleration ink-jet recording apparatus is known which performs the slit jet recording method. This recording apparatus comprises an ink-ejecting slit having a width of about 100 .mu.m and a length of about 200 mm, a number of recording electrodes juxtaposed in the slit in such a density of about 8 pieces/mm, and means for applying high-voltage pulses to those of the recording electrodes which have been selected in accordance with the data to be recorded on a recording medium. Those portions of ink which are located near the selected recording electrodes applied with high voltage are attracted to back electrodes by virtue of electrostatic force. As a result, ink dots are formed on a recording member located between the ink-ejecting slit and back electrode, whereby data is recorded on the recording member.
A method of applying high-voltage pulses on the selected ones of the many recording electrodes is known, in which the recording electrodes are connected to high-voltage pulse generating circuits, respectively, and the pulse generating circuits are selectively driven in accordance with data to be recorded on a recording member. To effect this method, it is necessary to use as many high-voltage pulse generating circuits as the recording electrodes. The ink-jet recording apparatus, which employs this pulse-applying method is inevitably large and expensive, and thus not so practical.
In recent years, a new ink-jet recording method has been proposed (Jpn. Pat. Appln. KOKAI Publication No. 60-250962). In this method recording electrodes are connected to a first high-voltage applying electrode by photoconductive insulators of the recording electrodes and also to second high-voltage applying electrodes by fixed resistors, respectively. While a high DC voltage is applying between the recording electrodes and second high-voltage applying electrode, optical signals corresponding to data to be recorded are applied to the photoconductive insulators, thereby changing the potentials of the recording electrodes in accordance with the data.
This method utilizes the fact that a photoconductive insulator has its resistances varied in accordance with the amount of light it receives. With this method, however, it is impossible to change the potentials of the recording electrodes greatly enough, and it is difficult to control ejection of ink. This is because the photoconductive insulator interposed between the first high-voltage applying electrode and each recording electrode, and the fixed resistor interposed between each recording electrode and the corresponding second high-voltage applying electrode have a limited withstand voltage.
The above-described method applying high-voltage pulses to the selected ones of many recording electrodes has a problem other than that indicated above. The other problem is electric field interference between any adjacent recording electrodes (The Transactions of the Institute of Electronics, Information and Communication Engineers, Vol. J66-C, No. 1, p. 48, January 1983). In order to eliminate the interference, a voltage cannot be applied on the adjacent recording electrodes at the same time. It is necessary to apply a high-voltage pulse to every other recording electrode, every two other recording electrodes, or so, thereby performing split driving. The printing speed will inevitably decrease. In addition, petroleum-based, high-resistance ink must be used to prevent a current leakage through the ink filled in the gaps among the recording electrode. Due to such a current leakage, changes the physical properties of the ink, particularly the resistance change caused by a temperature change, would greatly vary the ejection characteristic of the ink. The narrower the gaps among the recording electrodes, the more prominent the current leakage. It is difficult to arrange more recording electrodes over a unit distance. That is, it is difficult to increase the density of the electrodes. High fidelity recording that can be attained by the slit jet recording method is inevitably limited.
The ink-jet recording apparatus, which is increasing in personal use, is of serial type which applies pressure on parts of ink in accordance with image signals, thereby ejecting ink droplets from an array of tiny nozzles.
The greatest problem with the ink-jet recording method of this is type that it is necessary to limit the number of tiny nozzles in practice due to ink clogging which may occur when the ink dries in the tiny nozzles. Ink clogging impairs the reliability of the ink-jet head is impaired. Thus it is much demanded that an elongated line head having high reliability be developed for the ink-jet recording method.
To solve the problem, various ink-jet recording methods have been proposed. Among these methods are: solid-state ink-jet method, wherein ink which remains solid at ordinary temperature is heated, melted and ejected through an array of tiny nozzles under pressure generated in accordance with image signals; thermal ink-jet method, wherein a thermal head heats a porous film containing ink, thereby ejecting the ink from the film; and above mentioned slit jet method (Jpn. Pat. Appln. KOKAI Publication No. 49-62024), electrostatic attraction is applied to ink, thereby ejecting the ink through a common ejection slit toward a common electrode, by using electrostatic attraction.
A requirement which the ink-jet recording method needs to meet is to record images at high resolution. Generally, high-resolution printing must be attained in order to reproduce a curved-line image with fidelity. The ink-jet recording reproduces images having a resolution of about 300 dpi, whereas electrophotography, which is widely used in offices, provides images having a resolution of 800 dpi. Obviously, ink-jet recorded images are inferior in quality to electrophotographed images. To raise the resolution of the ink-jet recording, nozzles must be arranged in a higher density. However, nozzles cannot be arranged so densely with the existing manufacturing technology, without degrading the reliability of the ink-jet head. As a matter of fact, the recording method has yet to achieve a sufficiently high resolution.